The question_dataset_dependent_questiontype was defined so that variable elements could be used in questions.
However only LITERAL are used and they are used to contain formulas giving numerical values in the so-called question_calculated_qtype.
In the processing the question_calculated_qtype values are transformed in numerical values and process as question_numerical_qtype what is called the $virtualqtype.
On the long term given that calculated was the only qtype using datasetdependent there was some functions treating calculated questions parameters that were placed in abstractype.php or that are redundant to calculated/questiontype.php.
This cleaning is done and all the datasetdepent properties preserved so that it could be used in the future.
However as this is an "abstractype" there will be no more rendering functions in the abstractqtype.php but the actual interfaces to modify the datasets (_form.php) will be left in the datasetdependent directory.